This invention relates to superconductors in general and more particularly to a method for establishing an electrical contact with a magnet winding in which a contact element consisting of normally conducting material is connected in an electrically conducting manner with the end section of at least one superconductor of an intermetallic compound which is formed by annealing an intermediate conductor product in-situ.
Conductive intermetallic compounds of the type A.sub.3 B with an A15 crystal structure such as for instance, Nb.sub.3 Sn or V.sub.3 Ga, have good super conduction properties and are distinguished by high critical values. Conductors with these materials are, therefore, suitable, especially for superconducting magnet coils, to generate strong magnetic fields. Besides the superconductive binary compounds mentioned, ternary compounds such as niobium aluminum germanium (Nb.sub.3 A1.sub.0.8 Ge.sub.0.2), are of special interest for conductors of such magnets.
However, these intermetallic compounds are, in general, very brittle, so that their manufacture in a form, for instance, for magnet coils presents difficulties. Therefore, special methods have been developed, by which such conductors with an A15 crystal structure can be made in the form of long wires or ribbons. In these methods, which, in particular, permit the manufacture of so-called multicore conductors, a first component of the intermetallic compound to be prepared which is a ductile element in wire form is generally surrounded by a jacket which consists of a ductile carrier metal having an alloy containing the other elements of the compound. For instance, a niobium or vanadium wire is surrounded by a jacket of a copper-tin bronze or a copper-gallium bronze. A multiplicity of such wires can also be embedded in a matrix of the corresponding alloy. The structure of these two components so obtained is then subjected to a cross section reducing process. One, therefore, obtains a long, wire-shaped structure such as is required for coils, without the occurence of reactions which would embrittle the conductor. After the cross section is reduced, the intermediate conductor product of a superconductor, consisting of one or more wire cores and the surrounding matrix material, is then subjected to an annealing treatment in such a manner that the desired superconductive compound with an A15 crystal structure is formed by a reaction of the core material with the further element of the compound contained in the surrounding matrix. The element contained in the matrix then diffuses into the core material consisting of the other element of the compound (see British Pat. No. 1.280.583).
Superconducting magnet coils made of such superconductors are generally manufactured by two different methods. In the first method, which is also called "react first, then wind" method, the intermediate conductor product of the superconductor to be manufactured is wound on a temporary coil form and is subjected then to the required annealing treatment to form the desired superconductive compound. Subsequently, the superconductor so produced is unwound again from the temporary coil form and can be processed further. In this connection, especially when winding magnet coils, the danger generally exists that the brittle intermetallic compounds of the conductors will be damaged due to excessive deformation and their super-conduction properties will be impaired accordingly.
These dangers do not exist in the second method for manufacturing the superconductive compound from the intermediate conductor product. In this method, which is also called the "wind and react" technique, the coil form of the magnet to be provided with the winding is first wound with the not yet reacted intermediate conductor product of the superconductor and, then, the entire magnet, so wound, is subjected to the diffusion anneal. This anneal is also called an "in situ anneal". With this procedure, all difficulties of processing a brittle conductor material are avoided. It is also possible in this manner to fabricate coils with small inside diameters and relatively thick conductors. However, with this method, all material used in the construction of the coil must withstand, for several hours to days, the high temperatures which are required for diffusion annealing, which can, for instance, in the case of niobium-tin, be 700.degree. C.
In such in-situ annealed magnet coils, the preparation of terminal contacts presents difficulties. For, because of the great brittleness of the reacted A15 superconductors, it is practically no longer possible to bend the leads after the in-situ anneal. Therefore, the ends of the intermediate conductor product of these wires can be arranged on specially formed contact elements of normal conducting material only prior to the in-situ anneal (Proc. of 6th Int. Conf. on Magn. Techn. (MT-6), Bratislava, CSSR, Aug. 29-Sept. 2, 1977--page 998). In the subsequent anneal, however, a reactive element of the conductive compound to be manufactured, for instance, the tin can diffuse out of the conductor material of the intermediate conductor product into the copper of the contact element. Thus, a depletion of the one component of the superconductive compound occurs. The results are weaker superconductor zones and thereby, reduced current carrying capacity of the superconductors. Further difficulties can occur at sharp bends. While this generally does not matter in the unreacted intermediate conductor product, such portions can become critical after annealing because of the mechanical stresses which occur there and are difficult to visualize. Such stresses are caused particularly due to materials of different shrinkage qualities located in the contact area.